The present invention relates to media used in electrophotographic printing and more particularly to a plastic sheet or film for use in electrophotographic printing comprised of a transparent or opaque polymeric substrate coated with a polymeric coating having particular hardness and glass transition temperature parameters.
Laser electrophotography is an important electronic non-impact printing technology. It has several advantages over traditional mechanical impact printing techniques, such as high resolution, low noise level and high speed. However, currently available receptor media for laser printers, particularly for desktop laser printers, do not provide satisfactory image quality. They are frequently deficient in toner adhesion and resolution and in providing uniformly dense characters. The present invention overcomes these problems in laser electrophotography. While laser electrophotographic printing is specifically discussed in the present specification, the invention is equally applicable to other exposing radiation such as light emitting diode (LED), liquid crystal shutter (LCS) and the like techniques. The laser electrophotographic process normally creates images on a coated polymeric substrate in five steps: charging, imaging, developing, image transfer and fixing. The individual steps of the process generally include the following:
(1) The electrophotographic process begins when a uniform electric charge is deposited onto a photoconductor drum in the dark;
(2) An electrostatic latent image is then created on the photoconductor by exposing the photoconductor to an oscillating narrow laser beam that is turned on and off digitally;
(3) The photoconductor is then exposed to toner particles, wherein toner particles having the correct polarity adhere to the exposed latent image;
(4) The medium to be printed is then passed between the photoconductor and a transfer corona to cause the toner particles to transfer from the photoconductor to the medium; and
(5) The transferred toner particles are then fixed to the medium by one of various procedures known in the art.
The last two steps in the imaging process represent difficult problems in the electrophotographic printing process. Although transfer of toner particles to the receptor layer is primarily driven by electrostatic forces, suitable transfer and fixing of toner particles to the receptor layer depends substantially upon the properties of the receptor medium. First, in order to insure the fidelity of the image transfer, toner particles must interact weakly with the photoconductor and strongly with the medium. Then, the receptor layer must be able to receive the toner particles completely in order to insure good image resolution. Finally, in the fixing process which follows image transfer, the toner particles must have a good affinity to the receptor layer in order to achieve image bonding.
The receptor sheet used in the process must also meet various important criteria. Most importantly, the receptor sheet surfaces must have suitable surface properties to insure reliable transport through the printer.
One of the most common problems in electrophotographic printing involves stoppage or delays resulting from jams due to inappropriate surface properties of the medium. In fact, if the imaging medium does not pass through the printer, none of the other qualities is relevant. Other factors include resistance to tearing and sufficient thermal stability to avoid buckling and loss of planarity.
Although various recording media have been proposed for use with laser electrophotographic printers, none of them has satisfied the substantial need in the art, particularly for laser printers such as the HP LaserJet and Apple LaserWriter. Current commercial laser media have two main drawbacks. First, the toner cannot be transferred fully from the photoconductor to the receptor layer due to poor toner affinity to this layer. Such incomplete toner transfer creates hollow characters and poor image resolution, both of which are considered as being serious quality defects. Secondly, images can also abrade or flake off from the medium because of poor toner adhesion. Other problems relating to imaging, medium handling and aesthetics are also encountered.